Processes for the conversion of iron into steel

ABSTRACT

Pure oxygen is blown into the molten bath through the bottom of the converter. During all or part of this refining process, a strongly oxidising gas, such as pure oxygen, is injected through the side wall of the converter below the upper level of the metal.

I United States Patent 1 [111 Coets Apr. 2, 1974 PROCESSES FOR THE CONVERSION OF 75/46; 266/41 IRON INTO STEEL [75] Inventor: Christian Coets, [56] Reerences Cited Vauxsous-Chevremont, Belgium UNITED STATES PATENTS 73 Assignee: Centre De Recherches} 3,706,549 l2/l972 Knuppel '75/6O Metallurgiques centrum voor 2,875,037 2/1959 Wright 75/46 Research In De Metallurgie, Brussels, Belgium Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg [22] Flled: 18, 1972 Attorney, Agent, or Firm-+-Holman & Stern [21] Appl. No.: 218,683

[57] ABSTRACT [30] Foreign Application priority Data Pure oxygen is blown into the molten bath through the Jan 1 8 1971 L b 62434 bottom of the converter. During all or part of this reuxem 0mg fining process, a strongly oxidising gas, such as pure oxygen, is injected through the side wall of the con- 75/60, 75j5z3ic7gg9t verter below the pp level of the metal. .[58] Field of Search 75/43, 59, 60, 41, 53, 11 Claims, 5 Drawing Figures PAIENTEUAPR 2 I914 3.801. 304

sum 1 or 4 l 15 t(min) primary auxiliary FIG.2.

PATENTEDAPR 2 I974 4 3801304 sum 2 OF 4 v YATENTEDAPR 21974 $801,304

' sum 3 OF 4 FIG PATENTED APR 2 I974 SHEET BF 4 FIG.5.

PROCESSES FOR THE CONVERSION OF IRON INTO STEEL The present invention relates to improvements in methods for converting iron into steel, particularly in the case where such conversion is. effected by blowing pure oxygen in through the bottom of the converter.

it is a well known fact that the problem of injecting, through the bottom of the converter, in the form of commercially pure oxygen, the quantity of oxidising gas necessary to refine a bath of hot metal has recently been the subject of work which provided a satisfactory solution to the problem of refractory life. In the techniques thus developed, injection is effected through tuyeres which take the form of two coaxial tubes, the centre tube serving as a duct for the oxygen and the outer tube for a gaseous or liquid fluid affording protection.

However, as steel makers well know, processes for refining iron which have involved blowing pure oxygen in through the bottom have hitherto made it difficult to achieve a genuine control of the development of the composition of the slag during the steel making process. It is known that such control is bound up with the possibility of controlling the foaming of the slag, the speed of carbon elimination and the distribution of the oxygen blown in between the metal and the slag. Now in refining processes in which pure oxygen is blown in through the bottom there is only one variable which can be utilised, viz. the flow of oxygen, variations in which by the nature of things can only have a very restricted influence on the behaviour of the slag.

The present invention has specifically as its object improvements in conventional processes for the refining of iron by blowing oxygen in through the bottom of the converter through suitable tuyeres such as tuyeres composed of two coaxial ducts, which improvements largely make it possible to reduce the disadvantages above mentioned.

The principle of the improvements consists in judiciously combining the blowing in of pure oxygen through the bottom of the converter and side blowing, preferably also of pure oxygen, so as on the one hand to increase as far as possible useful variations in the rate of flow and on the other to assist oxidation of the slag by the oxygen thus blown in.

By means of this process it is possible to control in a much more thorough manner during refining the distribution of the oxygen blown in between the metal and the slag since there are two sources from which oxygen is blown, one of which, the bottom, is able to act more on the metal while the other, side source, acts more on the slag.

A non-limiting example of the refining of hematite iron in a converter with a perforated bottom equipped with tuyeres at the side will demonstrate how the process may be used.

FIG. 1, shows, for a hematic iron, the general graph for the elimination of silicon (curve 1) the decarburization rate (curve 2), and the integral of the rate of addition of CaO (curve 3), the abscissa being graduated in minutes.

FIG. 2, which has the same abscissa as FIG. 1, shows very diagrammatically on a conventional scale the time graphs for the blowing of oxygen into a converter. Curve 4 relates to bottom blowing and curve 5 to side blowing.

At the beginning of the refining process, the oxygen is blown in almost only, through the bottom, causing combustion of the Si, the commencement of decarburization and the combustion of a certain amount of Fe which contributes to the formation of slag, with the CaO supplied from above. During this phase, little oxygen reaches the slag. When the greater part of the silicon has been burnt out, the oxygen begins to be injected through the side so that the oxygen injected through the bottom may be used almost completely to burn out the carbon. Since the oxygen injected through the side reaches the slag more easily it provides the slag with the oxygen necessary for it to remain in the desired fluid and oxidised state, despite the addition of lime. Furthermore, it markedly reduces the risks of slag overflowing.

Once the lime has all been fed into the furnace, the oxygen requirement of the slag drops and it is possible to reduce the amount of oxygen blown in through the side and increase that blown in through the bottom as far as possible so as to complete the carbon elimination while maintaining the slag in the state desired.

This example shows how by means of the process of the invention it is possible during refining to control the quantities of oxygen injected through the bottom and the sides so as to distribute the oxygen between the metal and slag in the manner desired and cause the slag to develop in a certain manner.

In practice, this side injection may be effected in a number of ways.

According to a first embodiment of this improvement, side blowing is effected below the upper level of the metal and is directed obliquely upwards with a pressure at most sufficient for the oxygen to reach the metal-slag interface.

According to this method, the said jets are so directed that the upper peripheral portion of the metal is that most affected by the injection. The oxidising reactions which are preferably produced there have the effect of causing a local peripheral boiling on the surface of the bath, which may take on a slightly concave upwardly curved profile. Since the slag is lighter than the metal it tends to move continuously towards the centre, this movement being, of course, encouraged by the actual peripheral injection. The oxidising reactions are thus caused to take place continuously on substances which are always being renewed.

I According to another particularly advantageous method, the side injection is effected obliquely and upwards in a direction substantially tangential to the inside walls of the converter so that the bath and then the slag floating thereon are caused to rotate rapidly. Under the effect of the centrifugal force which has thus appeared, the upper surface of the metal may become curved and more or less concave in an upwards direction while the slag, being lighter, collects towards the centre. Advantage may then be taken of the fact that the slag is nearer the bottom of the converter to increase the force of the jets of oxygen blown in through the bottom and thus penetrate at least as far as the vicinity of the slag-metal interface. A loss of oxygen as a result of the excessive force of the jet is not a risk because of the greater thickness of the slag at that point. It should be noted that this bottom blowing operation, which is preferably carried out through the centre of the bottom, hardly disturbs the rotary movement of the bath, firstly because it is central and secondly because the rotary force exerted by the side jets is directed upwards.

It is also possible, according to the invention to reach the slag-metal interface or even the slag itself with oxygen blown in from the side and directed towards the centre of the converter so as to be almost horizontal (at a very slight vertical thickness) so as not to further to disturb the rotation of the moving mass of metal. It does not, of course, exceed the scope of the invention to cause rotation of the bath of metal first in one direction and then in the other so as to regulate the wear and tear on the lining.

Depending upon the case, the refining oxygen blown in through the bottom'and the sides may hold in suspension slagging and fluidising materials, ore, lime etc., liquid or gaseous hydrocarbons, water vapour, and neutral or inert gas either singly or as a mixture.

It has also been found advantageous to carry out the above-described refining processes in a converter in which the internal shape of the lining assists the sliding of the metal in an upwards direction. In this respect it is of advantage to have a converter which has at least in its lower portion an upwardly directed marked recess, since the slag then has an increased tendency to collect and become very thick at the centre of the bath and at a relatively low level.

It does not exceed the scope of the invention to combine the above processes with the additions of any solids fed into the top of the converter.

It is possible to combine bottom blowing with a double tangential injection, one in each direction, in order to cause a double swirling movement in the bath without appreciably affecting the shape of the bath surface. This mode of procedure, for example, with an inert gas such as CO would make it possible to obtain at the end of the refining process, by agitating it thoroughly, a bath of metal which is particularly homogeneous. It goes without saying that when one or other of the tuyeres is not to discharge oxygen for a time, it is supplied with a suitable gas at a pressure sufficient to prevent its blocking up.

Purely by way of example, the following drawings show how'one might conceive of a converter for use with the method described above.

FIG. 3 is a vertical section through the converter; FIG. 4 is a cross section at the level of the horizontal injection tuyeres and FIG. 5 is a cross-section at the level of the tangential tuyeres. In these figures like numbers represent like parts.

In FIG. 3 the converter 6 with axis 7 is filled with metal to be refined 8, on which floats a slag 9. In the bottom of the converter 10 there is arranged a blowingunit 11 containing tuyeres 12 through which oxygen is blown into the bath. The lower side wall 13 of the converter takes the form of a cone which diverges in an upward direction. This wall is provided, below the upper level of the metal (at rest) with a series of plugs 14 (FIG. 4) provided with tuyeres l5 directed towards the centre and substantially horizontal. Below the level of the plugs 14 there are arranged two plugs 16 (FIG. 5) each provided with two groups of tangential tuyeres l7 and 18 so that oxygen may be blown into the bath impart cause it to rotate in either direction or to impact to it a double swirling movement.

FIG. 3 shows, at 19 and 20 and in very diagrammatic form, the shape that might be assumed by the upper surface of the metal and slag if the speed of rotation of the bath were to become high enough. As described above, injection through the bottom (tuyeres 12) is mainly designed to transfer oxygen to the metal while the side injections through tuyeres 14, 17 and 18 are mainly designed to convey oxygen to the slag, the method of the invention being as described above, and consisting in adjusting either injection in order to control the development of the refining process. Any change in the shape of the bath under the influence of centrifugal force due to rotation or peripheral injection is designed to bring the slag nearer the tuyeres and thus facilitate contact between oxygen and the slag.

I claim:

I. A process for refining iron in a converter having a bottom wall and a side wall portion extending upwardly from the bottom wall, said convertercontaining a molten bath of metal on which floats a slag, which comprises blowing pure oxygen through the bottom wall of the converter and, during at least part of the time during which oxygen is blown in through said bottom wall, injecting a strongly oxidizing gas as an auxiliary injection through the side wall of the converter below the upper level of the metal, said auxiliary injection of oxidizing gas reaching the metal-slag interface.

2. The process of claim I wherein CaO is supplied to the converter and wherein oxygen is initially blown in only through the bottom until the greater part of the silicon has been burnt out of the iron, then the oxidizing gas is introduced into the converter through the side wall along with the oxygen which is blown in through the bottom until the CaO has all been fed into the converter, after which the flow of oxidizing gas through the side wall is reduced and the flow of oxygen through the bottom is increased.

3. A process as claimed in claim I, wherein the distribution of the oxygen between the metal and the slag is controlled by controlling the injection of oxygen through the side and the bottom.

4. A process as claimed in claim 1, further comprising imparting to the metal and slag a rotary movement about the converter axis, the converter being stationary.

5. A process as claimed in claim 1, wherein said auxiliary injection jet is directed upwards towards the axis of the converter.

6. A process as claimed in claim 1, wherein said auxiliary injection includes substantially horizontal jets which are directed towards the axis of the converter and slightly upwards.

7. A process as claimed in claim 1, wherein the auxiliary injection includes jets directed substantially tangentially to the side wall.

8. A process as claimed in claim I, wherein, during all or part of the refining operation, slagging and fluidising materials, are added.

9. The process of claim 4 wherein the rotary movement is caused by the auxiliary injection itself.

10. The process of claim 4 wherein the bath is subjected to such a speed of rotation that, under the effect of centrifugal force and gravity, the level of the metalslag interface in its central portion drops sufficiently to allow the auxiliary injection to cross this interface and penetrate into the slag.

11. The process of claim 4 wherein the rotary speed imparted to the bath is sufficient to lower the level of the slag-metal interface in its central part under the effect of gravity and centrifugal force and wherein the flow of oxygen blown in through the bottom is increased so as to reach at least the vicinity of the slagmetal interface in said central part. 

2. The process of claim 1 wherein CaO is supplied to the converter and wherein oxygen is initially blown in only through the bottom until the greater part of the silicon has been burnt out of the iron, then the oxidizing gas is introduced into the converter through the side wall along with the oxygen which is blown in through the bottom until the CaO has all been fed into the converter, after which the flow of oxidizing gas through the side wall is reduced and the flow of oxygen through the bottom is increased.
 3. A process as claimed in claim 1, wherein the distribution of the oxygen between the metal and the slag is controlled by controlling the injection of oxygen through the side and the bottom.
 4. A process as claimed in claim 1, further comprising imparting to the metal and slag a rotary movement about the converter axis, the converter being stationary.
 5. A process as claimed in claim 1, wherein said auxiliary injection jet is directed upwards towards the axis of the converter.
 6. A process as claimed in claim 1, wherein said auxiliary injection includes substantially horizontal jets which are directed towards the axis of the converter and slightly upwards.
 7. A process as claimed in claim 1, wherein the auxiliary injection includes jets directed substantially tangentially to the side wall.
 8. A process as claimed in claim 1, wherein, during all or part of the refining operation, slagging and fluidising materials, are added.
 9. The process of claim 4 wherein the rotary movement is caused by the auxiliary injection itself.
 10. The process of claim 4 wherein the bath is subjected to such a speed of rotation that, under the effect of centrifugal force and gravity, the level of the metal-slag interface in its central portion drops sufficiently to allow the auxiliary injection to cross this interface and penetrate into the slag.
 11. The process of claim 4 wherein the rotary speed imparted to the bath is sufficient to lower the level of the slag-metal interface in its central part under the effect of gravity and centrifugal force and wherein the flow of oxygen blown in through the bottom is increased so as to reach at least the vicinity of the slag-metal interface in said central part. 